UAE4ALL on RPi does not detect Joystick Fire

Hello,

UAE4All works well on raspberry Pi2 (thank you, it's great !), with an USB Joypad.
It seems the working button for fire is the index 2 (zero-based index), determined with the "jstest" linux command (for example "jstest /dev/input/js0").

But I now use a real Amiga Joystick with a DB9 to USB converter, and the fire button is detected as the button index 0 :

Axes: 0: 0 1: 0 Buttons: 0: on 1: off 2: off 3: off 4: off 5: off 6: off 7: off

Fire button works in EmulationStation menus, but it isn't detected at all in UAE4ALL Amiga Games (Dynablaster, Pang, Battle Squadron).

My question is : how could I make the fire button 0 working in UAE4ALL ?
- is it possible to map the button 0 to button 2 in the Linux layer ? (already done in retroarch, but config is UAE4ALL independant).
- would the solution be to modify the UAE4ALL code to make the button 0 to be copied to button 2 (in the bit mask in "od-joy.cpp") ?
- I don't know if I could do a DB9 to DB9 adapter since the third button seems to be a little bit special (I am not allowed to post a link, see "DB9 amiga joystick" first answer on Google). And I would prefer to find a software solution.
- or is this a planned evolution or a known bug in UAE4ALL ?

For information, the button 0 works fine in WinUAE on Windows. So detecting the button 0 does not seems to be abnormal.
I contacted the adaptator supplier, which told me he respected the USB HID standard.